The present invention relates generally to circuits and techniques for providing an accurate indication of the present “depth of discharge” (DOD) of a rechargeable battery, and particularly to such circuits and techniques that avoid the need to fully charge and then immediately fully discharge the battery in order to obtain an accurate DOD value in spite of fade of the full charge capacity of the battery in the course of repeated use.
It is well known that a no-load measurement of the terminal voltage of a battery (or cell thereof) is an accurate indication of its DOD if the battery has been in a no-load condition for a substantial amount of time, which may be from approximately half an hour to several hours. It also is well-known that an estimate of the “remaining run-time” of a battery operated device is dependent on accurate knowledge of the presently available energy that remains stored in the battery, and that the maximum energy storage capacity of a battery decreases, i.e., “fades”, substantially with the “age” of the battery.
Note that the “age” of a battery is generally understood to refer more to the number of charge/discharge cycles to which the battery has been subjected than to the actual amount of time that the battery has existed. In some applications, the battery capacity fade with age must be determined in order to provide accurate estimates for the amount of energy presently stored in the battery, so that the remaining run-time of the battery-powered device can be accurately determined. The presently known methods of determining the amount of battery capacity fade are based on a complete discharging of the battery from a fully charged condition.
However, it is often inconvenient to the user to ensure both that the battery is initially fully charged and then is immediately fully discharged as is required for the prior methods of determining battery capacity fade. Typically, such a full discharge occurs only by chance, and rarely (or never) happens in actual use of the battery to power a device. Instead, most battery operated systems such as electronic notebooks, laptop computers, mobile phones, personal digital assistants (PDAs) and the like do not provide continuous heavy loading on the battery, but instead are characterized by short periods of high current drain on the battery and long periods of relative inactivity in “off” or “standby” conditions.) Consequently, battery capacity fade usually is not properly determined during the long intervals between the rare occurrences wherein the battery is fully discharged immediately after being fully charged. Therefore, the accuracy of the terminations of remaining run-time become less and less accurate as the battery “ages”.
Although certain corrections of battery capacity calculations can be performed numerically using previously measured data representing the dependency of battery capacity fade based on the number of prior charge/discharge cycles of the battery and the amount of total cumulative charge that has passed through the battery, such corrections require time-consuming collection of data that is specific to each particular model or type of battery. Furthermore, such data usually is quite imprecise because battery capacity fade is heavily dependent on the various conditions of the battery usage, such as temperature, frequency of charging/discharging, etc. An additional complication is that the battery voltage measurement used to determine when the discharge of the battery is complete is heavily dependent on variable loading of the battery, and that corrections therefore require techniques that are time-consuming and expensive because the corrections need to consider transient loading effects and battery-specific data. These considerations all make it very difficult to provide the estimates of battery capacity fade that are needed for generating an accurate determination of remaining run-time of the battery powered device.
Attempts have been made to use voltage under load to determine remaining capacity of the battery. However, that requires correction for voltage drop due to internal impedance of the battery. Due to complex nature of internal impedance, voltage drop generally depends on the battery relaxation time since the most recent load change, which makes this method very imprecise in the case of variable loads. Additionally, precise knowledge of battery impedance is needed for such correction. However, battery impedance varies as much as +−15 percent even in newly manufactured battery cells. The battery impedance also depends strongly on temperature (about 1.5 times the change with 10 degree Centigrade change of temperature) and increases by a factor of roughly 2 during first ⅕ of battery life, but the amount of this increase can not be precisely predicted. All of these elements of uncertainty make the estimation of remaining battery capacity based on battery voltage under load very imprecise and no longer satisfactory for the rapidly growing reliability needs of users of battery powered mobile devices.
Thus, there is an unmet need for an improved battery measurement system and method that provides an accurate present estimate of battery capacity despite fade in battery capacity due to aging so as to allow generation of an accurate indication of remaining run-time of a battery powered device.
There also is an unmet need for an improved battery measurement system and method that reduces or eliminates the need for collecting data for each particular battery model in order to provide a basis for accurate determination of battery capacity for the purpose of determining remaining run-time run-time.
There also is an unmet need for an improved battery measurement system and method that avoids the need for correction for battery self-discharge in order to accurately determine battery capacity.
There also is an unmet need for an improved battery measurement system and method that avoids the large uncertainty associated with low rate discharge in determining battery capacity.
There also is an unmet need for an improved battery measurement system and method that avoids the need for providing a full battery charge followed by a full discharge in order to obtain an accurate determination of battery capacity.
There also is an unmet need for an improved battery measurement system and method that allows the state of charge determination to be based on a single small database suitable for most batteries of a particular chemistry.
There also is an unmet need for an improved battery measurement system that does not rely on correction for the voltage drop due to internal battery impedance, which is subject to large uncertainties due to transient effects and is not capable of precise estimation.